1. Field of the Invention
The present invention pertains to remote sensing, and, more particularly, to a Doppler beam sharpening technique for use in a remote sensing system.
2. Description of the Related Art
A significant need in many contexts is to locate and determine the position of things relative to some point. For instance, in a military context, it may be desirable to determine the position or to locate an object relative to a reconnaissance or weapons system so that the object may be targeted. In World War II, the British developed and utilized radio detection and ranging (“RADAR”) systems for remotely sensing the relative position of incoming planes of the German Luftwaffe. RADAR uses radio frequency (“RF”) electromagnetic waves to detect and locate objects at great distances even in bad weather or in total darkness. More particularly, a RADAR system broadcasts RF waves into a field of view, and objects in the field of view reflect the RF waves back to the RADAR system. The characteristics of the reflected waves (i.e., amplitude, phase, etc.) can then be interpreted to determine the position of the object that reflected the RF wave.
Some RADAR systems employ a technique known as “Doppler beam sharpening” (“DBS”). DBS uses the motion of an airborne RADAR to induce different Doppler shifted reflections from different cells on the ground. For a fixed range the cells have different Doppler frequencies because each is at a different angle relative to the source of the RADAR wave. This angle comprises depression and azimuth components in rectangular coordinates or, in polar coordinates, a “look angle.” Thus, projections of the RADAR's velocity on each cell differ, thereby allowing for discrimination of each from the other. Azimuth resolution comes from the Doppler frequency, while range is retrieved from pulse delay. Azimuth resolution is related to Doppler filter bandwidth which is inversely related to the integration time of that filter—the aperture time.
Consider, for instance, the scenario 100 depicted in FIG. 1. A platform 103 is traveling in a direction defined by the vector V defined in an X-Y-Z Cartesian coordinate system. The platform 103 may be an airborne vehicle, such as an aircraft, a reconnaissance drone, a missile, or a guided submunition, or may be a spacecraft, such as an orbiting satellite. The platform 103 is equipped and is using a conventional DBS RADAR system transmitting, e.g., RF waves 106-109, into a field of view. The field of view is, in this particular scenario, a cone defined by the platform 103 and the footprint 112. The footprint 112 may be, for instance, an area on the ground painted by the RF waves transmitted by the DBS RADAR system. For ease of illustration, the footprint 112 is shown in the X-Y plane of the Cartesian coordinate system. The vector V′ is a projection of the vector V onto the X-Y plane.
The footprint 112 comprises a number of cells, or sub-areas, such as the cells 115-118. Each of the cells 115-118 is at least a slightly different distance from the platform 103, i.e., their ranges from the platform 103 vary. Each of the look angles θ1-θ4 for the waves 106-109 relative to the direction of travel V is also at least slightly different. The characteristics of the reflections of the waves 106-109 are products of these two facts. For instance, the travel time from the platform 103 to the cells generates a “pulse delay” in the reflection relative to the respective wave 106-109 of which it is a reflection. Thus, the magnitudes of the pulse delays are a measure of the range to the cells 115-118. The look angles θ1-θ4 impart what is known as a “Doppler shift” into the frequency of the reflection, the amount of the Doppler shift being a function of the magnitude of the angle.
The DBS RADAR system, upon receipt of the reflections, then processes the reflections to extract information such as the pulse delay and Doppler shift that they contain. From this information, the DBS RADAR system can generate an “image” of the footprint 112. The image is a data set representative of the content of the footprint 112. The pulse delay provides resolution, or detail, about the content of the footprint 112 for this image in range. The Doppler shift provides resolution in azimuth. The magnitude of reflected signal from each ground cell is proportional to a pixel grayscale value in the resulting image. FIG. 2 is a highly stylized depiction of a visual display 200 of such an image, including a land mass 203 and a body of water 206.
However, DBS RADAR systems have range dependent resolution and a blind zone dead ahead of the DBS RADAR's motion. A blind zone 209, centered on a boresight 212, is shown in FIG. 2. The blind zone 209 pictured might be a meter across or a mile. The magnitude is unknown as there is no range reference in the drawing. The blind zone 209 results because, ahead of the platform 103, there are insufficient differences in the Doppler shift generated by the cells for the DBS RADAR system to distinguish them. More technically, DBS RADARs have problems pulling cells out of fields of view directly ahead of flight because, for a given resolution, any separation between adjacent iso-Doppler curves becomes too narrow. That is, the iso-Doppler contours get too close together for a fixed resolution and to resolve them requires ever-narrower filters compared to broadside ground-cells.
The reflections sometimes present what are known as “Doppler ambiguities” in the filed of view where the field of view encompasses both sides of the boresight. The ambiguities arise because not only are the iso-Doppler contours too close together, cells close to the boresight and the same distance off the boresight will have the same returns. That is, close to the boresight, the returns from cells equidistant from the boresight are indistinguishable. This causes ambiguities during processing because it cannot be determined from which side of the boresight a return came.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.